Control system for fluid cat cracker



May 19, 1970 J. H. SMITH CONTROL SYSTEM FOR FLUID CAT CRACKER 2Sheets-Sheet 1 Filed Aug. 50, 1968 REACTOR STEAM I8 Th P 1 Tl-l FRESHFEED 4 RECYCLE I u & S C O m WA W T I 2 E w W R V Y N H R I R N w m m 3s J T 1 T A N O R .r H m 7+. i R C F wm llllllllllllll I! m a .Ewwm u SI E J R C A O R A C P .|||I II E N R W P A W 6 T P l..| H F N E G E R P\FUEL 9 F/G 7 ATTORNEY May 19, 1970 J. H. SMITH 3,513,087

CONTROL SYSTEM FOR FLUID CAT CRACKER Filed Aug. 30, 1968 2 Sheets-Sheet2 E t j E 65 a 2- REACTOR TEMP. FRESH FEED RATE FIG? F/G. 4

RECYCLE RATIO CATALYST LEVEL IN REACTOR F/G. 3 Fla. 5

INVENTOR. JOHN H. SMITH M (7 flip/ m ATTORNEY United States Patent OU.S. Cl. 208-459 Claims ABSTRACT OF THE DISCLOSURE Coke deposition ratein the reactor of a fluid cat cracker is controlled by varying theseverity and conversion level in the reactor, while maintaining maximumair rate to the regenerator. The temperature at the outlet of theregenerator, which is an indication of afterburning and amount of cokebuild-up, is used to control reactor temperature and catalyst-to-oilratio in the reactor (by resetting the reactor temperature recordercontrol, which in turn controls the flow of hot regenerated Catalystfrom the regenerator to the reactor), thus controlling coke depositionto make it commensurate with air supply by controlling the severity andconversion level in the reactor.

DISCLOSURE In a fluid cat cracker it is important that the air rate tothe regenerator at any instant be commensurate with the rate of cokedeposition on the catalyst. When the air supply is insuflicient, theconcentration of coke on circulating catalyst in reases rapidly becausea dirty catalyst is less eflicient, the rate of coke deposition thereonbeing a direct function of coke concentration thereon. In revivifyingcatalyst by burning coke therefrom in the dense phase in a regenerator,typically onethird or more of the carbon in the coke is converted r toCO rather than CO Provided the normal regenerator temperature equals orexceeds the ignition temperature of CO (as it typically does) andprovided an excess of air is used for regeneration, CO will burn to COin the upper section of the regenerator above the dense catalyst phase.This phenomenon, known as afterburning, causes a rise in temperature ofthe flue gases above the dense phase in proportion to the amount of COconverted to CO If a large excess of air is used in the regenerator, thetemperature rise due to afterburning can be sufiicient to cause severedamage to the upper part of the regenerator and the flue gas dischargecircuit. On the other hand, a small amount of afterburning withconcomitant low temperature rise can be an excellent control guidebecause it indicates that the air rate is suflicient to balance the cokedeposition rate Without being in wasteful or damaging excess.

Historically, operators of fluid catalytic crackers have variedregeneration air rate to counterbalance coke deposition rate. Thisnecessitates operating at Conditions which consume less than maximumavailable air at all "ice times to provide leeway for moderate increasesas well as decreases from minute to minute as coke deposition rateincreases or decreases. In recent years, some refiners have resorted tothe installation of very elaborate and cost- 1y computer control systemsto vary severity of the cracking operation to take fuller advantage ofthe avialable air supply. Still, they must waste some air for trimcontrol. As a direct result, their coke burning rates are lower than themaximum potential. In turn, they convert less gas oil than the maximumpotential capacities of their units, nearly all of which are aircapacity limited.

SUMMARY OF THE INVENTION My invention is a simple low-cost scheme ofautomatic or semi-automatic control which balances coke deposition rateto air supply rate. This permits the owner of a fluid cat cracker totake full advantage of his installed air compression capacity at alltimes even though this capacity meanders with changes in atmosphericpressure, temperature, and humidity. This control is accomplished byvarying reaction severity to adjust conversion upward or downward asvariations in air supply permit the burning of more or less coke. Thereaction severity is controlled by varying the temperature and thecatalyst-to-feedstock ratio in the reactor in response to thetemperature at the outlet of the regenerator. The temperature and theCatalyst-to-feedstock ratio is varied by varying the flow of hotregenerated catalyst from the regenerator to the reactor.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a simplified flow diagram of afluid catalytic cracking unit embodying my invention.

FIGS. 25 illustrate the relationship of certain operating variables toprofitability of a conventional fluid catalytic cracking unit.

DETAILED DESCRIPTION Referring to FIG. 1, compressor 1 runs at maximumspeed on governor control, delivering all the air it can compress intoregenerator 2 in which the pressure is held constant (preferably atabout 1030 p.s.i.g.) by a pressure recorder controller (PRC) 3aoperating a back pressure valve in the flue gas exit line 3. Fresh feedand recycle gas oil streams 4 and 5 each on flow rate control (PRC) 4aand 5a, join and pass through furnace 6 into the reactor feed riser 7.The transfer temperature typically varying between about 400 F. and 700F. from feed preheater furnace 6 is controlled by a temperature recordercontroller alarm (TRCA) 8 which positions a valve in the furnace fuelline 9. The temperature in the dense phase 10 of the regenerator is heldconstant by a temperature recorder controller (TRC) 11 which resets thecontrol point of TRCA 8.

Steam and slurry recycle from the fractionator (not shown) may also becharged to the reactor feed riser at constant rates as indicated.Regenerated catalyst from the dense phase in regenerator 2 flows via astandpipe 12 and slide valve 13 into the lower end of the reactor feedriser 7 where it mixes thoroughly with the steam and oil feed streams.By virtue of its higher temperature (about 1200 F.), the regeneratedcatalyst surrenders heat to the combined oil feed stream, bringing it tothe desired temperature (usually within the range of 890 F. to 960 F.)to effect vaporization and cracking of the latter. The resultant vaporsand steam flow up through the reactor feed riser into reactor 14,entraining the catalyst therewith. The temperature within the reactor iscontrolled to maintain a reactor temperature preferably within the rangeof about 890-960 F., by a temperature recorder controller alarm (TRCA)15 which regulates the position of the slide valve 13 in the regeneratedcatalyst standpipe 12.

The amount of afterburning which occurs in regenerator 2 is controlledby controlling its flue gas exit temperature with a temperature recordercontroller (TRC) 16 which resets the control point on TRCA 15 whichdirectly controls the reactor temperature.

Spent catalyst from reactor 14 gravitates through stripper 17 wherein itis countercurrently swept with steam fed to the base of the stripper vialine 18 con trolled by a flow recorder control (FRC) 19. This steamstripping removes adsorbed and entrapped oil vapors from the spentcatalyst and returns them to the reactor from whence they ultimatelyflow to the fractionator.

Stripped spent catalyst gravitates from the base of stripper 17 intoregenerator 2 via spent catalyst standpipe 20 and slide valve 21. Theposition of slide valve 21 is regulated by a level recorder controller(LRC) 22 to maintain a constant head of catalyst above the base ofstripper. The catalyst level may extend up into the reactor if desired.

Having thus explained the system of controls, it is appropriate toelaborate on how and why it works. Referring to FIG. 2, it will be seenthat, at constant coke burning rate, the overall profitability of atypical fluid catalytic cracking operation does not vary significantlywith reactor temperature provided the temperature is held in themid-range, which for a mid-continent gas oil feed will usually be about900 F. to 925 F. As is well known, this essentially constantprofitability range varies with feedstock characteristics and variousunit and refinery factors, increasing with increasing aromaticity andnitrogen content. At excessively low or high temperatures, higher cokeyield is realized per unit of conversion; hence the conversion capacityof any given unit declines as reactor temperature approaches eitherextreme.

Referring to FIG. 3, it can be seen that increasing recycle ratioincreases cracking efiiciency (volume of gasoline yield per volume ofgas oil converted), but with steadily declining effect. Since the unitcost of recycling is essentialy constant, the overall profitability as afunction of recycle ratio goes through a maximum with very little changefor some distance on either side of the maximum.

In the typical refinery, profitability of catalytic cracking increaseswith fresh feed rate as shown in FIG. 4, without limit except foravailability of feedstock or cat cracking capacity.

Among the variables which effect the level of conversion are reactiontemperature, catalyst to oil ratio, catalyst activity, and catalystinventory in the reactor. Of these, catalyst inventory in the reactor isthe least desirable variable for forcing higher conversion. This hasbeen discussed at great length in numerous publications, and theconcensus is that high catalyst inventories in cracking reactors leadsto recracking of gasoline fractions and other deleterious reactions.Thus, as indicated by FIG. 5, profitability typically increases withdecreasing catalyst level in the reactor without limit provided limitingconversion can be reached in the reactor feed riser without excessivetemperature or catalyst cost.

Except for very short term deviations, the heat input to a catalyticcracker must equal the heat output; otherwise system temperatures mightrise or fall to damaging levels. The sum total heat input via the feedpreheater 6 and via combustion within the regenerator 2 must equal thesum of the radiation losses, the sensible heat surrendered to the fluegas leaving the regenerator, and the sensible, latent, and reactionheats surrendered to the product vapors from the reactor 14.

If, at any time, the temperature of the dense phase 10 in theregenerator should tend to drop, it would signify that the rate of heatremoval from the system temporarily exceeded the rate of heat input; theregenerator TRC 11 would react by increasing the control setting of thefeed preheater TRCA 8. This, in turn, would increase the fuel rate (line9) to the furnace 6 to bring heat input and output back into balance andreturn the regenerator temperature to its control point. It is readilyapparent that by reacting in opposite fashion, the control system justdescribed will limit the degree to which the regenerator temperature canclimb above the control point.

The control temperature setting on the TRC 16 must be somewhat higherthan that on TRC 11 to insure controlled afterburning. This differenceshould be at least 5 F. to insure reasonable controllability but shouldnot be so high as to be wasteful of air that might be better used forburning additional coke which would result from raising conversion. Insome instances it might be desirable to operate with a flue gas exittemperature 50 F. or more above the regenerator dense phase temperatureto maintain a high mean oxygen concentration in the gases rising throughthe dense phase to reduce the residual coke content on regeneratedcatalyst to a lower level than would otherwise be achieved. For thiscontrol scheme to function properly, it is obviously necessary that thecontrolled temperature level in the regenerator dense phase exceed theignition temperature of carbon monoxide. The preferred temperature isabout 1200 F. to 1225 F.

If, at any time, the temperature of the flue gas exiting from the top ofregenerator should tend to fall below the control point, it wouldsignify that there was a reduction in afterburning because of a drop inoxygen content of the flue gases rising from the dense phase. This, inturn, would signify that the mean concentration of coke on catalyst inthe regenerator was rising which would mean that coke was beingdeposited at a faster rate than it was being burned. The flue gas TRC 16would immediately lower the control setting on the reactor TRCA 15 whichwould, in turn, re-position (partially close) the slide valve 13 in theregenerated catalyst standpipe 12. The combination of lower reactiontemperature and lower catalyst-to-oil ratio would reduce coke depositionrate by reducing conversion level until coke deposition rate againbecame commensurate with regeneration air rate (line 1a). It is readilyapparent that if the flue gas exiting temperature should tend to rise,it would signify that coke was being deposited at a lesser rate than itwas being burned and that the automatic control action would be theexact opposite of that just described to increase conversion rate untilthe coke deposition and burning rates again became equal.

The feed preheat furnace TRCA 8 includes high and low temperature alarmswhich alert the operator if the temperature reaches either alarmsetting. The operator then takes action to reduce or increase heat inputrequirement of the system. For example, if the low temperature alarmshould sound, the operator may increase recycle rate (line 5) to removemore heat from the system to the fractionator. He might take theopposite action if the high temperature alarm should sound.Alternatively, TRCA 8 might be equipped with a reset mechanism toautomatically reset the control setting of the recycle FRC 5a if eithertemperature alarm point should be reached.

In actual operation, the system may be simplified further bysubstituting potentiometers or other temperature sensing means for TRC16 and TRC 11 and allowing the operator to manually reset TRCA 15 andTRCA 8, respectively.

The reactor temperature control 15 includes high and low temperaturealarms which alert the operator if the temperature reaches either alarmsetting. The operator then takes appropriate action to bring the reactortemperature back within the prescribed range. For example, if the hightemperature alarm should sound, he would take some action to increasethe severity of some reaction control variable other than temperature.This might be an increase in reactor catalyst level, catalyst activity,or recycle ratio, or a reduction in dilution steam rate to the riser orregenerator dense phase temperature. If the low temperature alarm shouldsound, the operator would take some action opposite to those justdescribed. Alternatively, the reactor temperature control might beequipped with one or more reset mechanisms to automatically effect oneor more of the changes indicated.

Although the control system could be made more complex as indicated inthe last two paragraphs, I prefer the simple version as described anddepicted in FIG. 1, relying upon the operator to take appropriate actionto keep the reactor and furnace transfer temperatures within the alarmsettings.

Although mid-continent gas oil is the only feedstock specificallymentioned above, this plan of control will improve the resultsobtainable with any feedstock otherwise suitable for catalytic crackingand will be especially beneficial for any feedstock that varies inquality during operations.

Suitable catalysts are conventional fluid catalytic cracking catalysts,which are well known in the art.

The first fluid catalytic cracking unit went on stream in 1942. Sincethen hundreds of units have been built around the world. Literallythousands of engineers have worked on this process at some time orother; yet apparently none has heretofore thought of this simple schemeof control which permits maximum conversion at all times as limited byair supply. Most units are operating 5 percent or more below capacity.Even those units equipped with complex and costly digital computercontrols are operating below the capacities obtainable with the simpleinexpensive control scheme described herein.

For example, a test run in a commercial unit substantially as shown inFIG. 1 was conducted using the control system of this invention. Thefeedstock was a midcontinent gas oil. During this test run,potentiometers were used in place of TRC 16 and TRC 11, the operatormanually resetting TRCA and TRCA 8, respectively, as needed. The unitwas operated using the maximum available air rate from the compressor(about 27,200 standard cubic feet per minute). Operation of the unit wascontrolled, in accordance with this invention, so as to balance the cokedeposition rate with the coke burn-ofl rate and at the same timemaintain the regenerator temperatures within proper limits. The maximumair rate to the regenerator prior to the use of this system had beenabout 25,300 s.c.f.m. (representing 92.5% of the maximum air rateavailable from the compressor), while achieving a gas-oil conversion of75.6 percent. With the use of this invention during the test, 100% ofavailable air (approximately 27,200 s.c.f.m.) was fed to regenerator (anincrease of 7.5 percent) while achieving a gas-oil conversion 79.2percent.

What is considered new and inventive in this present invention isdefined in the hereunto appended claims, it being understood, of course,that equivalents known to those skilled in the art are to be construedas within the scope and purview of the claims.

I claim:

1. In the continuous process of cracking a hydrocarbon feedstock in thepresence of subdivided catalyst particles, wherein the hydrocarbonstream effects a fluidized contacting of the particles in a reactor,conversion products are separated from the contacted particles,separated catalyst particles containing coke deposited thereon efi'ectfluidized contacting of air in a separate regenerator, said air beingsupplied by an air compressor, combustion gas products are separatedfrom regenerated catalyst particles and such regenerated catalystparticles with a reduced coke content are returned to the reactor forcontact with hydrocarbon feedstock, the improvement which comprises:operating the air compressor at maximum capacity with all of the airoutput going into the regenerator, and controlling the coke depositionrate in the reactor by varying the reaction severity in the reactor inresponse to variations in the air supply from the compressor.

2. The process of claim 1 in which the reaction severity in the reactoris controlled by varying the temperature and the catalystto-feedstockratio in the reactor in response to the temperature at the outlet of theregenerator while the temperature in the regenerator catalyst bed isheld constant.

3. The process of claim 2 in which the temperature and thecatalyst-to-feedstock ratio is varied by varying the flow of hotregenerated catalyst from the regenerator to the reactor.

4. In a catalytic cracking unit consisting essentially of: i

(a) A reactor having a first conduit attached to the upper portionthereof, a stripper attached to lower portion thereof, and a temperaturerecorder controller alarm attached to said reactor;

(b) A regenerator having attached to the upper portion thereof a secondconduit and means to control the pressure in the regenerator, and havingattached to the lower portion thereof a third conduit, equipped with avalve, said third conduit extending up into the lower portion of theregenerator, and a fourth conduit connected to the lower portion of theregenerator, said fourth conduit having an air compressor connectedthereto;

(0) Means for controlling the operation of said valve in response totemperature variations in the reactor;

(d) A conduit connecting the lower portion of the reactor with thebottom end of said third conduit and extending beyond said connectionand through a furnace fired by means of fuel supplied to said furnacethrough a fifth conduit;

(e) Means connected to the regenerator and the fifth conduit formaintaining a substantially constant temperature within the regenerator;

the improvement which comprises:

(f) Temperature sensing means connected to the upper portion of theregenerator, said temperature sensing means being operatively connectedto said means (0).

5. The combination of claim 4 wherein said temperature sensing meanscomprises temperature control means for changing the control setting ofmeans (c) in response to temperature variations in the upper portion ofthe regenerator.

References Cited UNITED STATES PATENTS 2,409,751 10/1946 Gerhold et a1.208-163 3,206,393 9/1965 Pohlenz 208-164 3,213,014 10/1965 Atkinson etal. 208-164 3,238,122 3/1966 Hagerbaumer 208-165 3,316,170 4/1967Stewart et al. 208-164 3,410,793 11/1968 Stranahan et al 208-1593,238,121 3/1966 Parkin 208-165 3,261,777 7/1966 Iscol et al 208-1133,004,926 10/1961 Goering 252-417 HERBERT LEVINE, Primary Examiner US.Cl. X.R.

